This invention relates generally to catalysts for facilitating oxygen reduction reactions such as those which occur at an oxygen reduction cathode in a metal- or hydrogen-fuel cell, and, more specifically, to methods of producing such catalysts.
Fuel cells, including without limitation metal- and hydrogen-fuel cells, are attractive alternatives to traditional energy source such as batteries and diesel generators since they can be refueled, do not consume fossil fuels, and do not give off noxious emissions into the atmosphere.
Typically, the cathode in such a fuel cell is an oxygen reduction cathode at which oxygen can be reduced according to the following reaction:
O2+2H2O+4exe2x88x92xe2x86x924OHxe2x88x92
To facilitate this reaction, and to avoid the formation of undesirable byproducts such as peroxide, it is desirable to provide an oxygen reduction catalyst at the situs of the cathode.
In one embodiment, the invention provides an oxygen reduction catalyst comprising (a) about 95 wt % to about 99.9 wt % carbon black, and (b) about 0.1 wt % to about 5.0 wt % metal, and (c) about 0.05 wt % to about 5.0 wt % nitrogen. In one configuration, the catalyst comprises a collection of oxygen reduction catalyst particles which particles are distributed in reasonably close proximity to each other. The collection may comprise no less than about 5 grams of the particles. In addition to particles, other configurations are possible such as where the catalyst is in the form of fibers, or in planar form. In one example, the catalyst which is produced is a carbon black supported catalyst in particulate form, which, in one non-limiting exemplary application, can be mixed with a binder and used to form the active layer of an oxygen reduction cathode for a fuel cell.
In a further embodiment, the invention provides a method for the high-throughput production of oxygen reduction catalyst. Again, the catalyst may be in any form, including, without limitation, particles, fibers, or planar form.
In one implementation, catalytic sites can be formed through the subject method on the surface of carbon black. To form these catalytic sites, one or more suitable metal-containing and/or nitrogen-containing precursor(s) is introduced to the carbon black. Optionally, a reducing agent can be mixed with the metal-containing and/or nitrogen-containing precursor(s) and/or the carbon black to enhance the formation of the catalytic sites. The reducing agent helps to provide a reducing atmosphere in the temperature range of between about 600 and 1000 degrees Celsius. These precursor(s) and the agent can be introduced in the form of a gas, a liquid (e.g., fine or micro droplet(s)), a solid (e.g., microparticles), or the like. The mixture of precursor(s), optional reducing agent, and carbon black can be heated to a temperature of between about 600 and 1000xc2x0 Celsius for a time between about 5 seconds and 240 minutes. Under these conditions, catalytic sites, in the form of a combination of the nitrogen, the metal, and carbon, will form on the surface of the carbon black.
In one embodiment, this high-throughput production method can be practiced at a production rate of no less than about 10 grams per day, with additional embodiments of a production rate of no less than about 100 grams per day and/or of a production rate of no less than about 1000 grams per day being contemplated for use in accordance with the present invention. Typically, the production levels suitable for use in accordance with the present invention will be in the range(s) from about 10 grams per day to about 100 kilograms per hour. Various embodiments for performing this high-throughput production method are possible, including continuous, semi-continuous, intermittent, and batch processes.
In another embodiment, a mixture of a carrier gas, a nitrogen-containing gas, and optionally, a reducing gas, can be caused to continuously, semi-continuously, intermittently, or in a batch mode (one-time), flow through a furnace in which a reaction zone is maintained at the desired temperature of between about 600 and 1000 degrees Celsius. The gas flows along a flow path that extends through the reaction zone, exits the furnace, and (optionally) re-enters the furnace. The carrier gas is substantially chemically inert in relation to the carbon black particles and metal precursor. The metal precursor comprises a metal salt. The carbon black particles and metal salt are placed in a liquid solvent, such as water, whereby the metal salt is absorbed by the carbon black particles. The mixture is then atomized to form a suspension of fine droplets, and the suspension introduced into the flow of the gas mixture. The gas mixture carries the suspension through the reaction zone one or more iterations until the cumulative time that the suspension has passed within the reaction zone is between about 5 seconds and about 240 minutes. Through this process, the liquid solvent evaporates, and catalytic sites form on the surface of the carbon black particles. At this point, the particles can be separated from the mixture, through filtering or the like, at the point where the flow path exits the furnace. The result comprises a carbon black backed catalyst in particle form.
Other systems, methods, features and advantages of the invention will be or will become apparent to one with skill in the art upon examination of the following figures and detailed description. It is intended that all such additional systems, methods, features and advantages be included within this description, be within the scope of the invention, and be protected by the accompanying claims.